The stellar rotation-activity relation for a sample of SuperWASP and ASAS-SN field stars

Abstract

It is well established that late-type main-sequence (MS) stars display a relationship between X-ray activity and the Rossby number, Ro, the ratio of rotation period to the convective turnover time. This manifests itself as a saturated regime (where X-ray activity is constant) and an unsaturated regime (where X-ray activity anti-correlates with Rossby number). However, this relationship breaks down for the fastest rotators. We cross-correlated SuperWASP visually classified photometric light curves and All-Sky Automated Survey for Supernovae automatically classified photometric light curves with XMM-Newton X-ray observations to identify 3 178 stars displaying a photometrically defined rotational modulation in their light curve and corresponding X-ray observations. We fitted a power-law to characterise the rotation-activity relation of 900 MS stars. We identified that automatically classified rotationally modulated light curves are not as reliable as visually classified light curves for this work. We found a power-law index in the unsaturated regime of G- to M-type stars of $β=-1.84 ± 0.18$ for the SuperWASP catalogue, in line with the canonical value of $β=-2$ . We find evidence of supersaturation in the fastest rotating K-type stars, with a power-law index of $β_s=1.42±0.26$.